The need to delay an optical beam in a repeatable manner is known. Apparatus for achieving such delay are commonly referred to as optical path delay scanners (or simply “delay scanners”). For example, a scanner may be used to control the phase of one beam of light relative to another beam of light. Such techniques find applications in fields of detection and measurement, among other fields.
For example, delay scanners are used for axial eye length measurements based on Michaelson interferometer arrangements. Generally, such apparatus include a beam splitter that 1) projects partially coherent light into a test arm onto a subject's eye, and 2) projects partially coherent light into a reference arm onto a moveable mirror. By moving the mirror a known amount and measuring an output including a combination of light reflected from the eye and from the mirror, portions of the output that are influenced by interference of light reflected from the eye and light from the mirror can be identified, and various eye lengths can be measured.
Numerous apparatus have been designed to implement such eye length measurement techniques. FIG. 1 illustrates an example of a conventional apparatus 100 in which light from a source 110 is projected onto a beam splitter 120 which projects light onto an eye E in a test arm 130 and onto a scanning minor 140 in a reference arm 150 to delay light in the reference arm relative to the test arm (as shown by arrow A).
An output signal from a detector 160 is generated by the combined reflections from the eye and mirror. The amplitude of the signal will increase and decrease due to interference (i.e., interference spikes will arise) when the length of the reference arm is within a distance equal to the coherence length of the light (as determined by the source) of a length in the test arm. A length of the test arm is determined by a reflective surface in the eye (e.g., a surface of the cornea or a surface of the retina). The amount which the mirror is moved between a location to achieve an interference output for a first surface (e.g., a surface of the cornea) and a location to achieve an interference output for a second surface (e.g., a surface of the retina) indicates the distance between the first surface and the second surface. An eye's overall axial length can be measured in this manner.
In apparatus as illustrated in FIG. 1, it is desirable that the delay in the reference arm (i.e., as caused by movement of mirror 140) occur in a highly linear manner. The linearity preferably is present over a substantial length (e.g., 15-35 mm) to obtain accurate eye length measurements. Because the mirror must be slowed in order to stop and reverse direction of the mirror, linearity at the end of the range may be insufficient. Additionally, because the mirror must be stopped and its direction reversed, there may be trade-offs between the repetition rate, duty cycle, delay magnitude and/or linearity that are achievable.
The time required to move the mirror can be substantial, particularly if multiple measurements of a given patient's eye length are to be made and averaged together. As a result, eye movement during measurement can be a source of error. While the speed of mirror movement can be increased, a drawback of increased speed is increased wear, vibration, and noise arising due to the stopping and starting of mirror movement.